One field of application for the invention is holding together a bone and a connection member in a desired relative position (while allowing in some cases a limited amount of relative movement), for example to aid in healing of breaks or to correct bony structure deficiencies and abnormalities. In particular, sufferers of abnormal spine curvature, spine deficiencies or other spine abnormalities (including scoliosis, instability of spinal segments or degenerated intervertebral discs) may benefit from the invention.
The spine is formed of superposed vertebrae, normally aligned along a vertebral axis, from the lumbar vertebrae to the cervical vertebrae, each having an anterior part: the vertebral body, and a posterior part: the vertebral arch (or neural arch). The anterior and posterior parts enclose the vertebral foramen. Each vertebral arch is formed by a pair of pedicles and a pair of laminae, with transverse processes and/or a spinous process (or neural spine) projecting therefrom. The transverse processes and the spinous process project opposite to the vertebral foramen.
When the vertebrae are articulated with each other, the vertebral bodies form a strong pillar for the support of the head and trunk. In between every pair of vertebral bodies, there is an intervertebral disc.
When the spine of a person has abnormal curvature (e.g. scoliosis) or other deficiencies (e.g. a damaged intervertebral disc), the vertebrae are typically too close together, too far apart or otherwise misaligned, and there is a need to stabilize the vertebrae in a correct position relative to one another. Mainly, there is either a need to compress the vertebrae (i.e. to bring and hold them closer together) or a need to distract the vertebrae (i.e. to move and keep them away from each other).
Typically, known stabilization devices include at least two anchors configured to be fastened, respectively, to two vertebrae, and a rod or other elongate member for connecting the anchors together, thereby providing stabilization between the vertebrae.
In one kind of device, known in the art, said anchors comprise a hook that rests on the vertebrae. Examples of such devices are disclosed, for instance, in PCT Application Publication No. 2005/023126, U.S. Pat. No. 4,269,178, U.S. Pat. No. 6,740,089, or US Application Publication No. 2007/0161990.
Compared to a pedicle screw, a hook has the advantage of providing a rigid and strong anchoring to the vertebra, but the physician (or other operative) may have difficulty in placing the hook correctly on the vertebra. Indeed, the position of the hook on the vertebra is unstable (i.e. the hook may slip out of place) until the rod or other elongate member is positioned and connected to all the anchors. Moreover, it is possible in some systems that the hook may disengage from the vertebra. Accordingly, the physician may decide to use more hook anchors than necessary in a spinal stabilization construct.
While known devices have proven effective, further improvements would be beneficial.